This invention relates generally to a heat exchange system, more particularly to a radiator design in a heat exchange system which utilizes a reciprocating electrochemical hydrogen pump, and even more particularly to a radiator design in a heat exchange system which utilizes a reciprocating electrochemical hydrogen pump and a liquid cooling agent that is mechanically isolated from the reciprocating electromechanical hydrogen pump.
Many devices must be cooled as they operate. Generally speaking, this cooling is effectuated by transferring heat from the device to be cooled, through a cooling agent of some sort, and finally to a thermally conductive heat sink. There are many different solutions for using heat sinks for the cooling of electronic devices and other parts. The heat exchange system must be designed to take advantage of the unique characteristics of the chosen cooling agent.
Electronic components are used in a countless array of devices to facilitate the faster processing of information. Although small in size, these devices and, more specifically, the integrated circuit chips that make up these devices, emit a tremendous amount of heat that is potentially detrimental to the chips themselves, the surrounding devices, and even the users of these electronic or other devices.
In a simple air cooled heat exchange system, the heat from be object to be cooled is transferred to the air by way of the surface area of a radiator. Radiators with different surfaces, sizes, and shapes of fins are selected depending on the amount of heat to be removed from the object to be cooled, the rate and direction of the air flow surrounding the radiator fins, and the temperature tolerance of the cooling device.
Likewise, the cooling of electronic and other devices utilizing a vapor compression refrigeration cycle is known in the art. Vapor compression cooling uses the thermodynamic principles associated with phase transfer, specifically the latent heat of vaporization and the entropy of evaporation of a working fluid. Compression of a vaporous working fluid can occur through mechanical, electrochemical, or other means. Mechanical compression requires a relatively large, heavy, mechanical compressor having a great number of parts which are often bulky and susceptible to wear. Because of these undesirable effects of using a mechanical compressor, electromechanical compressors have been proposed to drive Joule-Thomson refrigeration cycles. (See, for example, U.S. Pat. No. 4,593,534 which is hereby incorporated by reference in its entirety.) This type of compressor is preferred over a mechanical or other compressor because an electrochemical compressor contains no moving parts, is vibration free, and has the potential for long life and reliability.
When a liquid is used as the cooling agent in the heat exchange system, the approach employs a hollow-finned radiator containing a cooling liquid under forced convection. The liquid has a thermal conductivity coefficient which is 200-300 times greater than the thermal conductivity coefficient of a gas, and therefore the liquid coolant radiator system has significant advantages compared with using gas as the heat carrier.
Although thermodynamically efficient, the major problem with using a liquid cooling agent is that the liquid must be circulated through the heat exchange system to ensure that heat is properly transferred from the object to be cooled to the heat sink. Conventionally, an electrical pump is used to push the liquid through the heat exchange system. However, such an electric pump is noisy in operation and cannot operate for an extended period of time without maintenance.
As a solution, the use of an electrochemical pump to carry a beat transfer coolant in a heat exchange system was proposed in U.S. Pat. No. 5,746,064 and U.S. Pat. No. 5,768,906 which are both owned by Applicant and are both incorporated ill their entirety herein by reference. These patents describe art electrochemical pump that is based on an electrochemical cell.
The simplest electrochemical cell consists of at least two electrodes and one or more electrolytes. The electrode at which an electron producing oxidation reaction occurs is the anode. The electrode at which an electron consuming reduction reaction occurs is called the cathode. The direction of the electron flow in the external circuit is always from anode to cathode. In order to drive the electrolysis reaction, it is necessary to apply electric power to the cell electrodes. The electrodes are connected through the electrical leads to an external source of electric power with the polarity being selected to induce the electrolyte anion flow to the anode and the cation flow to the cathode.
Generally speaking, the anode and cathode are made of a substrate material, such as titanium, graphite, or the like, coated with a catalyst such as lead dioxide or other known materials. The selection of a substrate and catalyst is determined by the particular electrode reactions which are to be optimized in a given situation. As a rule, a cathode and an anode produce different products. Classically, these products are hydrogen and oxygen.
Generally, the electrolyte is a liquid which is conductive of ions. The most common applications are fuel cells. In fuel cells, proton exchange membranes are used as electrolytic and catalyst support for providing a reaction of hydrogen oxidation on the one side of the membrane and oxygen reduction reaction on the other side of the membrane. This type of electrochemical cell often produces wasteful water or gas which must then be carried away from the cell""s electrodes.
This hydrogen circulating through the heat transfer system from one electrode to the other can be used as the cooling agent in the system. As the gaseous hydrogen travels from one electrode to the other, it comes into thermal contact with the object to be cooled. The hydrogen gas cooling agent can come into direct contact with the object to be cooled or, more likely, the cooling agent will come into direct contact with a member that is in thermal contact with the object to be cooled. Heat will be transferred by this contact from the object to be cooled to the hydrogen gas. As the gas circulates, the gas will then come into contact with a heat sink or other heat well where heat will be transferred from the gaseous hydrogen to the heat sink, restoring the hydrogen""s original thermal properties. At the end of the circulation cycle, the hydrogen gas is used up in the oxidation reaction at the anode.
Hydrogen as discussed herein is very useful as a cooling agent. Hydrogen has a thermoconductivity value seventeen times that of air. However, hydrogen does have some limitations when compared to liquid cooling agents. For example, hydrogen has a low magnitude of specific capacity which may make hydrogen less appealing for larger volume applications. For larger volume applications, the system will be more efficient if the hydrogen is used as a pump for a liquid cooling agent.
Because of the small magnitude of specific thermal capacity of the hydrogen gas, a liquid cooling agent is often used in combination with the hydrogen pump. As such, the liquid cooling agent is inserted into the tube connecting the two electrodes in the electrochemical pump to each other. As the hydrogen circulates through the pump from cathode to anode, the gas pushes the liquid cooling agent through the tube also. In this system, it is the liquid cooling agent, not the hydrogen gas, that performs the heat transfer utilizing the benefits of greater thermal mass explained above.
The major problem with using the hydrogen and liquid cooling agent combination is that only the hydrogen is used up in the oxidation reaction at the anode. Hence, the hydrogen can keep circulating throughout the system, but the liquid cooling agent cannot do so. Hence, the hydrogen gas will compress the liquid cooling agent towards the anode end of the system as much as possible, but the system""s heat exchange properties are quite limited at that point in time. Hence, the electrochemical pump needs to be able to reverse itself so that the compressed liquid cooling agent at the former anode will flow back past the object to be cooled and into the gas space around the former cathode. Hence, to be effective, a heat exchange system utilizing a liquid cooling agent must be reciprocalxe2x80x94i.e.: it must be cyclic.
A solution to this problem was proposed in U.S. patent application Ser. No. 09/353,458 which is owned by the Applicant and is incorporated herein by reference. This application proposes an electrochemical pump that utilizes matching symmetrical hydrogen electrodes and a proton exchange membrane to produce an electrochemical pump utilizing a proton exchange membrane fuel cell. Such a cell""s electrochemical reaction can be reversed by reversing the polarity of the power source connected to the electrodes of the cell. The pump can produce hydrogen gas at either electrode, and if the reaction is cyclically reversed, the production of hydrogen gas, and hence, the flow of the liquid coolant through the system, can be cycled.
To effectuate such a cyclic heat exchange system, the power supply can be a reverse-polarity power supply capable of switching the direction of current applied to the electrodes which reverses the chemical reactions that take place at the electrodes. The poles of the power supply should be designed to be switched if a voltage difference across the hydrogen electrodes is within the range of about 100 mV to 600 mV for single cell. When liquid is used as a heat carrier, the polarity should be switched so that each of the electrodes alternatively produce and consume gas. The power supply may be a battery or a rectifier circuit or any other electric source capable of delivering direct current.
Such a reciprocal electrochemical pump has many advantages over a standard electric or other pump. The electrochemical pump has no moving parts and hence is almost completely noiseless. The electrochemical pump is very efficient in that it takes a small amount of power from the electric source to remove a substantial amount of heat from the object to be cooled. And finally, the electrochemical pump can be made that is very small in size and has little mass.
Electrochemical-based cooling system is do have drawbacks over air-cooled and other heat transfer systems. As explained above, an electrochemical pump must be used in a process in order to be utilized in a cooling system. The pump produces fluid motion by generating gas at one electrode. However, this gas only pushes the cooling liquid in one direction. Hence, after the cooling liquid is pushed in one direction, the electrochemical pump""s underlying reaction must be reversed so that gas can be produced at the opposite electrode thereby pushing the cooling liquid in the opposite direction. Such a system can be quite complicated in practice.
A second disadvantage of using an electrochemical pump as part of a heat exchange system is that water must be used as the cooling liquid. Vapor of the liquid used as the heat carrier penetrates in the pump area, condenses there, and adversely affects pump operation. To limit this effect, distilled water is used because the small amount of impurities limits the harmful effect on the electrodes. However, even if distilled water is used as the heat carrier, the condensation of the water vapor in the pump area creates a flooding problem for the electrode stack. Once flooding begins, it slows down hydrogen diffusion to the electrode which causes a decreased amount of hydrogen production. Such decreased amount of hydrogen production causes a decreased flow rate for the heat carrier water, which in turn causes a lower rate of heat transfer from the object to be cooled to the heat sinks. This makes the heat transfer system more efficient and decreases the object""s temperature.
In accordance with the present invention, there is provided a radiator design in an electrochemical heat exchanger which contains a gas-based pumping or driving system which is capable of forcing movement of a liquid-based cooling system by way of a flexible diaphragm located integrally between the two systems. Preferably the gas-based pumping system is comprised of a hydrogen electrochemical pump which is capable of producing hydrogen gas at one electrode and consuming hydrogen gas at the other electrode thereby creating a pressure differential between the two electrodes. Preferably, the liquid-based cooling system is comprised of first and second liquid spaces which are mechanically coupled to first and second banks of radiator tubes or fins respectively to transfer heat from an object to be cooled, through the liquid cooling agent, and out of the heat exchange system by way of the radiator fins or tubes.
Preferably, the two flexible diaphragms which are located between the gas pump system and the liquid cooling system mechanically isolate the contents of the two systems while keeping the contents of the two systems in pressure contact with each other. Pressure contact is intended to mean that a pressure on one side of the diaphragm from the contents of one of the systems will result in a deforming of the diaphragm and a resultant increase in pressure and decrease in volume on the contents of the other system at the other side of the diaphragm.